The research efforts of this laboratory center on documenting the organization and variation in the nervous systems of living vertebrates and utilizing these data to elucidate mechanisms underlying brain evolution. In this application, a plan is outlined for continuing studies on the organization and evolution of the vertebrate telencephalon. Living vertebrates constitute two major evolutionary radiations: jawless fishes (agnathans) and jawed vertebrates (gnathostomes), and thus far most experiments on the telencephalon have been confined to gnathostomes. Studies carried out on amphibians and lungfishes (gnathostomes) during past years of this project suggested several hypotheses regarding evolution of the vertebrate telencephalon that can be tested only by carrying out experiments on agnathans. Living agnathans--hagfishes and lampreys--exhibit extensive olfactory projections to much of the pallium. The olfacto-recipient pallium may include a dorsal pallial zone that in gnathostomes is a major site of ascending thalamic projections, or agnathans may possess an additional pallial zone that does not receive secondary olfactory input and may or may not receive thalamic input. In lampreys the dorsal thalamus may projection only to the striatum, whereas in hagfishes thalamic projections to both the striatum and portions of the pallium may exist. Analysis of the afferent and efferent connections of the pallial and dorsal thalamic areas in hagfishes and lampreys, as determined by horseradish peroxidase and tritiated amino acid tracing methods, and characterization of the distribution of various neuropeptides and biogenic amines, as revealed by immunohistochemistry, will provide the data needed to discriminate among the various hypotheses of telencephalic organization and their evolutionary implications.